This paper investigated the kinetics of the first two foot contacts after leaving the blocks in sprint running. Twenty-three international, national and recreational running sprinters performed maximal blocks starts. Data from 5 metre time and ground reaction forces from the first 2 strikes after exiting the blocks were recorded. The athletes were divided into 4 groups; senior male elite (SME), senior female elite (SFE), junior male elite (JME) and senior male recreational (SMR). The Kruskal-Wallis ANOVA with post hoc analysis was used to identify differences between the 4 groups. The SME group displayed the fastest time to 5m (1.41s), then the JME group (1.49s), next the SMR group (1.50s) and lastly the SFE group (1.55s). During the first 2 steps after leaving blocks the propulsive impulse accounted for between 95% and 98% of the total contact impulse. The anterior-posterior ground reaction data of the propulsive phase showed that the maximum propulsive force is larger (p<0.05) in the SME group when compared to the SFE, JME and SMR groups. The maximum propulsive force and 5 meter time display a high Spearman`s rank correlation for the 1st (r = 0.67) and 2nd (r = 0.62) steps. The maximum propulsive force was the most important component of the propulsive phase and correlates well with 5 meter time. To produce fast sprinting times over the first 5m it is imperative that sprinters produce a large propulsive force during the first 2 foot contacts, rather than putting too much emphasis on reducing braking forces.
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